The present invention relates to a cold-rolled steel strip with an electrodeposited nickel coating.
Cold-rolled strips of this type are employed in extremely diverse fields of application, where modern production techniques impose exacting requirements on the material with regard to its mechanical properties, surface finish, workability, and the like, and these requirements can be satisfied only by cold-rolled products. After cold deformation by means of appropriately prepared rollers, a cold-rolled steel strip conforming to DIN 1624 has surfaces that are smooth, dense, and bright, or that exhibit a uniform, slight roughening. In the RP and RPG surface-finish qualities, this cold-rolled strip is both pore-free and crack-free (with RPG quality also denoting a high gloss finish), so that the strip can be subjected to surface treatments with a view to increasing its corrosion resistance, in particular nickel-plating, without encountering any problems. Deep-drawable cold-rolled steel strips having an electrodeposited nickel coating are accordingly known.
In the electroplating of strip material, economic factors dictate that the coatings should in principle be thinner than those which are usually applied when electroplating individual articles. By adopting suitable measures, such as anode-screening arrangements, flooding, and the insertion of perforated plates in front of the anodes, it is possible to achieve conditions such that coatings of uniform thickness are deposited, with only minimal variations. However, the comparatively thin coatings suffer from the disadvantage of being less corrosion resistant than thicker electroplating. Furthermore, it is a drawback that a cold-rolled steel strip, whether electroplated or not, has a tendency to "stick" when it is annealed as a tightly-wound coil. Such "sticker spots" occur chiefly when cold-rolling a low-carbon strip having a surface that exhibits very slight micro-roughness. After coiling and heat treatment, "sticker spots" form, either sporadically or over large, connected areas, and at these spots the superimposed surfaces tenaciously stick to one another, and are difficult to separate. On being unwound from the pay-off reel, the "sticker spots" are separated by being torn apart, thus damaging or destroying the high-quality finish. Furthermore, "sticker spots" can cause serious interruptions in the operation of the production line, as well as give rise to material that has to be scrapped.
In the context of the production of coldrolled steel strips, it is known, moreover, that nickel-plated strips may be subjected to a heat treatment prior to further processing, with the purpose of this treatment being to increase the deformability of the composite system represented by the strip plus the coating. During this heat treatment, the electrodeposited nickel diffuses into the base metal. The diffusion rate is relatively low, which is disadvantageous, and the process is time-consuming and expensive if the aim is to achieve certain diffusion depths and to form solid solutions or mixed crystals of defined compositions.
It is therefore an object of the present invention to develop an electroplated cold-rolled steel strip which can be produced economically, and which exhibits no tendency to stick, has good deformability, has a deep coating diffusion, has advantageous corrosion behavior, and has improved electrochemical behavior, the development of this strip being designed to avoid the aforementioned drawbacks. It is a further object of this invention to provide a process for producing such a cold-rolled strip.
These and other objects and advantages of the present invention will appear more clearly from the following specification and the examples contained therein.